Various systems have been developed to steer machines. Traditional arrangements turn a front or rear wheel or pair of wheels to steer the machine while the remaining wheels follow. Another arrangement is utilized in so called “skid steering” in which the wheels on one side of the machine are driven at a different rate of speed or direction than the wheels on the other side.
Articulated machines include a front frame member, a rear frame member, and an articulation joint connecting the front and rear frame members. Typically, to steer an articulated machine, the front frame member must be rotated relative to the longitudinal axis of the rear frame member about the articulation joint while the machine is being propelled. To rotate the front frame member, these machines commonly include articulation cylinders mounted between the frame members. The articulation cylinders are operated in opposite directions so that when one cylinder is extended forward, the other cylinder is retracted rearward and vice versa causing the front of the machine to face the intended turn. U.S. Pat. No. 6,283,237 to Muller presented an alternative steering arrangement wherein rotation of the wheels on one side of the machine is at a speed greater than the wheels on the other side of the machine. The difference in the speeds causes the machine to articulate and steer the machine in a desired direction.
Machines having a broad wheel base with two pairs of rear wheels and a spaced pair of front wheels, as in a motor grader, present additional steering and operational challenges, whether the machine is articulated or not. For example, it can be difficult to make tight corners in a machine that has such a broad wheelbase. Traction may be lost on one or more of the wheels, resulting in wheel slip. When steering the front wheels in a desired turn direction, they may lean and/or skid, causing not only excessive, and/or uneven wear on the tires, but failure to execute a desired turn. Moreover, it may be difficult to maintain a relatively constant load on an associated tool, such as a blade, which is commonly utilized with a motor grader.
Accordingly, it is desirable to provide an improved steering system for machines such as a motor grader that includes two pairs of rear wheels, and a spaced pair of front wheels. The present disclosure is directed to overcoming one or more of the problems as set forth above.